This invention relates generally to gas turbine engines and more particularly, to methods and apparatus for cooling gas turbine engine rotor assemblies.
Turbine rotor assemblies typically include at least one row of circumferentially-spaced rotor blades. Each rotor blade includes an airfoil that includes a pressure side, and a suction side connected together at leading and trailing edges. Each airfoil extends radially outward from a rotor blade platform. Each rotor blade also includes a dovetail that extends radially inward from a shank extending between the platform and the dovetail. The dovetail is used to mount the rotor blade within the rotor assembly to a rotor disk or spool. Known blades are hollow such that an internal cooling cavity is defined at least partially by the airfoil, platform, shank, and dovetail.
At least some known high pressure turbine blades include an internal cooling cavity that is serpentine such that a path of cooling gas is channeled radially outward to the blade tip where the flow reverses direction and flows back radially inwardly toward the blade root. The flow may exit the blade through the root or the flow may be directed to holes in the trailing edge to permit the gas to flow across a surface of the trailing edge for cooling the trailing edge. In cooled turbine blades, the internal pressure of cooing air is attempted to be maintained greater than the local external pressure in the area of the blade. The amount by which the internal pressure exceeds the external pressure is typically referred to as positive Back Flow Margin (BFM). Having a positive BFM prevents hot gas ingestion into the blade interior in the event of a breached wall or severe cycle deterioration.
Furthermore, the aft tip region typically operates at an elevated temperature with respect to the rest of the blade such that film cooling in this area is desirable to improve blade life. In some known blades this film cooling is provided by using film holes in flow communication with a third or aftmost cavity in the cooling circuit. However, adequate internal pressure in the third cavity may not be able to be maintained in all cases. The second cavity or the cavity adjacent and upstream of the third cavity has adequate pressure but is located too far forward to be able to provide film cooling where it is needed.